The invention relates generally to wireless communications and, more particularly, to a feed network for simultaneous transmission of narrow and wide beams from a cylindrical antenna.
As mobile communications, such as wideband code division multiple access (xe2x80x9cWCDMAxe2x80x9d) and global system for mobile communications (xe2x80x9cGSMxe2x80x9d), proliferate, the number of antennas required to provide communications coverage increases. For a variety of reasons, it may be preferable to make these antennas xe2x80x9cconformalxe2x80x9d to some existing structure. For example, it may be aesthetically preferable or functionally necessary to unobtrusively mount a base station antenna on the wall of a building. Or, for aerodynamic reasons, an antenna mounted on an airplane would need to conform to the contours of the airplane. Conformal or, more generally, xe2x80x9cnon-planarxe2x80x9d array antennas offer the potential of an integrated, non-obtrusive solution for multibeam antenna applications. Two (2) basic xe2x80x9cconformalxe2x80x9d antenna geometries used for this are the circular-cylindrical and spherical array antennas.
The use of array antennas in mobile communications base stations has been shown to facilitate increased network capacity due to the creation of narrow (pencil or directional) beams that reduce interference levels. Narrow beams provide a xe2x80x9cspatial filterxe2x80x9d function, which reduces interference on both downlink and uplink. On downlink (i.e., from base station to mobile device), a narrow beam reduces the interference experienced by mobile devices not communicating via the beam in question. On uplink, a narrow beam reduces the interference experienced by the base station for communication links using the beam in question.
Vertically installed implementations of rotational-symmetric array antennas can offer omnidirectional coverage in the horizontal plane by the use of multiple beams. The beams are typically formed using the radiation from more than one (1) element (or vertical column) along the circumference of the array (i.e., the horizontal radiation pattern is an array pattern). For fixed-beam antennas, the individual elements (or columns) will be connected, via a feed network, to a number of beam ports. Each beam port generates the element excitation of one or (typically) more columns. An omnidirectional antenna can produce an omnidirectional pattern having essentially identical gain/directivity in all directions in a plane simultaneously. If a beam covers all 360xc2x0 in a given plane simultaneously, it is omnidirectional in that plane and there is no need to steer the beam. Omnidirectional coverage enables a communications link that is independent of the direction from the base station to the mobile unit. An omnidirectional pattern provides omnidirectional coverage at all times, whereas a pencil-beam (narrow beam) antenna with steered (or fixed) beams can provide omnidirectional coverage by directing (or selecting in the case of fixed beams) a beam in a desired direction. A steered (or selected) beam will only cover a portion of the desired angular interval at a given instant in time.
Although the generation of simultaneous pencil- and sector-covering beams is trivially achieved in the planar array case by placing a sector antenna next to an array antenna, a similar arrangement is not possible for a circular array. An extra sector antenna (i.e., an omnidirectional antenna) would have to be placed above or below the circular array in order to avoid interference with the array beams.
A number of feed networks exist which provide some, but not all, of the aforementioned capabilities. Although theoretically lossless and feeding all elements in parallel, an Nxc3x97N Butler matrix will generate N rotational-symmetric patterns, but without the pencil-beam shape. A Blass matrix is similar to a Butler matrix in that they both depend on directional couplers to achieve a desired distribution of power through the feed network. Although a Blass matrix can be used to generate pencil-beams, it cannot provide N identical beams due to the discontinuity of the element excitations when the network is used to feed a circular array.
Another class of feed networks is lenses. Lenses can be made to produce pencil-beams, but they suffer from loss due to non-orthogonality of the beam ports. Even if orthogonality can be achieved, lenses for omnidirectional coverage are typically unwieldy and expensive to manufacture, particularly as compared to transmission-line feed networks.
Therefore, no viable antenna feed network presently exists that can enable a rotational-symmetric array antenna to: (1) generate N identical fixed pencil-beams simultaneously, (2) generate each pencil beam using respectively corresponding antenna elements that are circumferentially separated from one another; and (3) generate an omnidirectional beam simultaneously with the pencil beams using the same antenna elements.
It is therefore desirable to provide a practical feed network that enables an N-element rotational-symmetric array antenna to generate N identical fixed pencil-beams simultaneously with an omnidirectional beam. In some embodiments, the present invention provides N identical fixed pencil-beams using fewer than N input ports of an Nxc3x97N Butler matrix that feeds an N-element rotational-symmetric array antenna, and simultaneously provides an omnidirectional beam by individually accessing one of the modes generated by the Butler matrix. The Nxc3x97N Butler matrix that feeds the array antenna can be driven by a feed network that applies both power division and beam-steering to a plurality of input beam signals, thereby permitting generation of N pencil-beams simultaneously.